This invention relates to an improved silicon carbide shape of the general type shown and described in U.S. Pat. No. 4,125,756. The shapes produced by the method described in that patent are densified by dipping them in furfural, subjecting the furfural coated shapes to fumes of hydrogen chloride, coated with silicon, and heated. It has been known for a long time to apply coatings of silicon carbide to shapes of carbon e.g. graphite, or of carburized or otherwise preliminarily coated refractory metals such as tungsten, by exposing the shapes to gas containing a halogenated silane such as methyltrichlorosilane. The prior art methods were suitable for deposition of silicon carbide on dense substrates, but not upon the porous, incompletely bonded granular body of the shape of the type described in U.S. Pat. No. 4,125,756. The prior art is represented by U.S. patents to Wainer, U.S. Pat. No. 2,690,409, and Clendinning U.S. Pat. No. 3,317,356, and British Pat. No. 955,700. Of these references, only the British patent suggests that silicon carbide might be suitable as a substrate. However, it is clear from the description in the British patent that a transparent, glassy, probably monocrystaline silicon carbide layer is to be deposited, and to this end, the British patent is emphatically specific to heating the shape itself to temperatures in the range of 1450.degree. to 1600.degree. C., before exposing it to the treating gas. For the production of resistance elements from the shapes produced by the impingement of a laser bean on a bed of unconsolidated particles of silicon carbide, this method is unsatisfactory. In accordance with the process of this invention, a polycrystaline, almost mud-like coating is provided, at deposition temperatures that are relatively low and deposition rates that are relatively high compared with the deposition temperatures and rates of the methods of deposition of monocrystaline material.
One of the objects of this invention is to provide a process of densification and subsequent nitriding that produces a more uniform, durable, and temperature stable shape from a partially bonded granular silicon carbide substrate than those known heretofore.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.